Fft Measurement - Agilent Technologies MSO6012A User Manual

5 Making Measurements

FFT Measurement

216
FFT is used to compute the fast Fourier transform using
analog input channels or math functions 1 + 2, 1 – 2, or 1 *
2. FFT takes the digitized time record of the specified source
and transforms it to the frequency domain. When the FFT
function is selected, the FFT spectrum is plotted on the
oscilloscope display as magnitude in dBV versus frequency.
The readout for the horizontal axis changes from time to
frequency (Hertz) and the vertical readout changes from
volts to dB.
Use the FFT function to find crosstalk problems, to find
distortion problems in analog waveforms caused by amplifier
non-linearity, or for adjusting analog filters.
FFT Units
0 dBV is the amplitude of a 1 Vrms sinusoid. When the FFT
source is channel 1 or channel 2 (or channel 3 or 4 on
4-channel models), FFT units will be displayed in dBV when
channel units is set to Volts and channel impedance is set to
1 MΩ .
FFT units will be displayed in dBm when channel units is
set to Volts and channel impedance is set to 50Ω .
FFT units will be displayed as dB for all other FFT sources
or when a source channel's units has been set to Amps.
DC Value
The FFT computation produces a DC value that is incorrect.
It does not take the offset at center screen into account. The
DC value is not corrected in order to accurately represent
frequency components near DC.
Aliasing
When using FFTs, it is important to be aware of frequency
aliasing. This requires that the operator have some
knowledge as to what the frequency domain should contain,
and also consider the sampling rate, frequency span, and
6000 Series Oscilloscope User's Guide